Please note, I'm not car savvy, including lingo, nor can I really fix stuff myself when it comes to cars. So keep that in mind!

2008 Volvo S40 with 219,816 miles. Beginning in 07/2020, I began having a check engine light come on and off that was identified to be code P0420. In 03/2021, I finally got it fixed when my local mechanic replaced our catalytic converter. Looking at that invoice, they also replaced the exhaust gasket, upstream and downstream oxygen sensors and replaced spark plugs.

Fast forward to now. On 2/10/23, my wife mentioned our check engine light came on and it's still on as of this message. This past 2/14/23, I took it to Autozone to get the code read and he said it showed code P0420. The rep there asked if we got gas recently and I told him yes, my wife got it the previous day on 2/9/23. He recommended simply changing out the gas cap to fix the problem. This is interesting to me because all that time from 7/2020 - 03/2021 when I finally replaced the CAT, I had always suspected and wondered if it was simply a gas cap issue. But I read information online with others shooting this idea down and say it would show a different code for that, etc. Is this simply an easy fix with the gas cap since we just replaced the CAT, etc. just two years ago? But even if it is just the gas cap, did I replace the CAT, etc. for no reason at all? My wife has mentioned the car doesn't feel any different driving since the light came on.

If this has nothing to do with the gas cap, I'm so lost and have no idea how to fix this problem. So confused and frustrated.

Thanks in advance for any help you can give.

 

I think the Autozone guy mixed up the "evap leak" code that can be set by a loose gas cap (ie a P0442 or a P0456) with what your P0420 code is reporting. The P0420 code says the rear oxygen sensor is detecting too much O2 after the catalytic converter. This can happen when there are exhaust leaks, issues with the O2 sensors, issues with fuel trim or a failing catalytic converter. Seems your repair job took on the exhaust leaks/sensors/fuel trim to kick the problem down the road a few years. At this point they can look for any air leaks or address a fuel trim issue, but the probable outcome may be you need to replace the converter. PS one way to "cheat" the code is to install a device called a "CEL-BOSS". Its a spacer that moves the rear sensor out of the airflow to reduce the amount of measured O2 to defeat the code. This may not be allowed in states with strict emissions testing rules.

 

Thank you for your reply! He seemed pretty confident understanding it was P0420 and the gas cap could be the issue though, but of course he could have been wrong like you mentioned. But do you not find it odd that my wife has just happened to get gas the previous day at a gas station we don't get gas at very often? I guess, I'm still confused though...you said the probable outcome may be that I need to replace the CAT. Why would I do that again if they just did that two years ago when I first had this P0420 code come up? Again, at that time they replaced the CAT, the exhaust gasket and the upstream and downstream oxygen sensors. It just doesn't make sense. In regards to the "cheat code", I'm not really sure how to do that nor do I feel comfortable since I'm not car savvy. Just don't feel comfortable but I appreciate the suggestion.

Just so lost on what to do next and how to proceed.

 

Here's the common logic on what you're seeing...

The original cats are very high quality (on most cars, and certainly on a Volvo), and are quite expensive to build since they contain a lot of very valuable materials (which is why they're targeted by thieves with battery operated saws).

Aftermarket cats can be fine, or can be total trash. FWIW, I put one on my '96 Jeep many (15?) years ago, and it's apparently still working just fine. Others can fail in months or years, or never work at all. With new sensors (again, assuming they used quality sensors) I'd suspect that's what's happened to your car. OTOH, if they used no-name sensors, they're suspect as well, and could be the cause of the problem. The good news is that a quality downstream (post-cat) sensor aren't horribly expensive. Here's a shot of the options (check to make sure I selected your car correctly - not sure if yours is a turbo or not)...

https://www.rockauto.com/en/catalog/...2)+sensor,5132

I'd go with the NTK or Bosch myownself...

 

Thank you again for another reply. According to my notes from the replacement of the CAT back in March 2021, it was a Bosal brand of CAT that apparently changed names to BRExhaust? Our mechanic told me that it is still considered OEM grade though and that it's almost exactly Magnaflow minus the sticker. They couldn't do Magnaflow for $400 more because they said it was a different bolt pattern for our car. Again, unfortunately I don't know much about cars so the option of buying something to fix this, I wouldn't even know how but I thank you anyways.

So no chance you really don't think this could just be a simple gas cap replacement? The owners manual even states that if you don't tighten the gas cap on well enough, it will trigger a check engine light? So what code would that be? My wife has mentioned that at times when she fills up gas, the cap seems loose when she is almost positive she tightened it pretty good when last getting gas. We also always get mid-grade gas after replacing the CAT two years ago. FYI, if it helps, the mileage when we got the CAT replaced was 184,232 and now we are at 219,816 when it happened so that is 35,584 miles that it lasted until this P0420 came up again two years later. In the past couple of years, we have also noticed lack of acceleration in building MPH when accelerating up hills, merging on to the interstate, etc. but we've brought it up a few times to our mechanic and they can't seem to figure out a problem that is causing that so we are left with no results. But not sure if that would have anything to do with this.

Just really unsure how to proceed.

 

Further discussion on the gas cap question. modern cars have an "evap" system that is designed to recover fuel vapors from within the gas tank. This design traps the vapors in a charcoal cannister (like a 6" box under the car back by the gas tank). There is an evap pump that actuates periodically to suck the vapors through the purge valve and into the intake. To do this properly, you need a sealed system (which is where the gas cap comes in). The OBD2 diagnostics can report when there is a "small evap leak" (such as a torn vacuum line or a poor gas cap seal) and a large leak (such as a failed charcoal cannister, problems with the evap pump or a completely removed gas cap. Now if the purge valve fails and remains open and your cap is off, you can effectively create an intake air leak which could lean out the mix - but its likely you'd see one of the fault codes for the evap system to suggest the purge valve needs to be tested in conjunction with a fuel trim or perhaps a cat efficiency fault code. There's no harm in periodically replacing the gas cap - in fact some state emissions test the gas cap seal. I just wanted to manage expectations that the likely outcome is the cat converter or related fuel trim / exhaust leaks are in play which is what your original tech worked on. +1 on the comments that some Volvos are very sensitive to the brand of cat - which is where a CEL-BOSS may be a viable option, particularly if your call is able to pass a tail pipe emissions test.

 

No relationship between a P0420 and a gas cap. The only similarity is that both can potentially turn on a CEL (Check Engine Light). You'll get an Evap(orative) system code if you have a bad / loose gas cap. Of course, if there's any doubt about how well your gas cap is sealing, get a new one... not worth risking a fail at your next inspection over a $10 cap.

Your symptoms (both driving and OBD code) are consistent with a clogged catalytic converter. That increases backpressure, which you'll notice first under high-load conditions (like the ones you mention), and will of course eventually result in a P0420 code as well).

This often happens after a long period of misfiring cylinder(s) or an oil-burning problem. If either of these are happening, fix that problem first, then move on to the cat.

The "quick fix" (or at least the easy half-hearted attempt at a fix) is to pour a bottle of "cat-cleaner" into your gas tank (following the directions, of course) and do some road miles, which theoretically can produce enough heat to allow the cleaner to restore the correct operation and flow through your cat. I'd suggest some research to find out which of these products work the best (I actually like the Amazon reviews for this, since there tend to be so many of them you don't have to worry about them all being from shills).

And finally, this is when it would be good to have an OBD reader. That way you can reset the codes after the above "fixes", and also check your "readiness codes". I believe that pretty much every state will pass you if you have only one or two "not ready" statuses on your readiness indicators - you'll want to check the statute and the readiness indicators on your car so you know when you can sail through inspection untouched. ;-)

 

Thank you both again for your replies and help on this. Just bummed it wouldn't be something like the gas cap....so the fact it seems looser when going to get gas doesn't change anything? I'm still just confused how this could be a bad catalytic converter again....this mechanic assured me it would last and said they never had issues with people coming back again after replacing them. So how am I that unlucky? Plus they said it was OEM grade? What assurance do I have that if I replace it again that this won't happen again? My details about lack of speed accelerating up hills, merging on interstate make you think even more that it's a bad CAT again and not the gas cap I take it? Also, if it's not the gas cap....why is there so many posts online about people reporting just having gotten gas right before their code P0420? Not disagreeing, just genuinely curious.

Also, if this is dealing with the CAT or something else besides the gas cap....how quickly do I need to get this checked out? Again, the light came on two weeks ago and has just been on the whole time since then. Unfortunately I don't feel comfortable trying some of the above methods with CAT cleaner, etc. with my limited knowledge on cars.